This is a guest post from Robert Brokamp of The Motley Fool. Robert is a Certified Financial Planner and the advisor for The Motley Fool’s Rule Your Retirement service. He contributes one new article to Get Rich Slowly every two weeks.

With the S&P 500 still down more than a third from its 2007 high, we’re all a little unsure about our retirement plans these days. So it’s time for some good old-fashioned elbow grease. A little effort now should make for a lifetime of security and peace of mind. And the first step is to run your numbers through financial calculators to estimate whether you’ll have enough saved to kiss the boss goodbye. (Metaphorically, of course.)

The calculators’ answers are important information. But what’s even more useful is changing the variables to see what most improves your chances for success. A retirement plan has a lot of moving parts — how much you save, where you live, when you start taking Social Security benefits — and some decisions will have a bigger effect on your nest egg than others.

The factors that have the biggest impact on a retirement plan vary from person to person. But to demonstrate how you can fiddle with your factors to analyze your own plan, let’s examine the retirement prospects of a hypothetical worker — whom we’ll call Hilda, as I’m a sucker for good German names — and see how dialing her numbers one way or the other changes her projected retirement income. Here are Hilda’s particulars:

  • Age: 55
  • Marital status: Single
  • Current income: $60,000
  • Desired retirement age: 65
  • Desired retirement income: $45,000
  • Estimated age at death: 95
  • Current savings: $100,000
  • Annual contributions to retirement accounts: $6,000
  • Assumed annual return on investments: 8%

For our analysis, we’ll use the “Am I saving enough? What can I change?” calculator found among the retirement calculators at The Motley Fool. Once you’ve entered your numbers and hit the “results” button, the calculator provides the number of months that it estimates your savings will last given your desired retirement income. In Hilda’s case, here’s the calculator’s analysis: “Your living expenses after retirement will be fully funded for 127 months.” Divide by 12, and you see that her money is expected to last 10.6 years. Unfortunately, that’s not good enough. If she wants to retire at 65 and expects to live until 95, she needs her money to last 30 years, or 360 months.

So what should Hilda do? Here are her options and possible outcomes, adjusted a single factor at a time.

Save More
Hilda’s current savings rate is 10% of her income. What if she ups that to 15%, or $9,000 this year? According to our calculators, that will make her income last 155 months — an additional 2.3 years. That’s a fine first step, but it still has her running out of money in less than 13 years.

Let’s say she, through a drastic lifestyle reduction, managed to contribute the maximum to her 401(k), which in 2009 is $22,000 for someone age 50 and older. That would supersize her portfolio enough to last an estimated 322 months, much closer to the 360-month mark. But she probably can’t save 36% of her income. She’ll have to look at other options.

Spend Less in Retirement
What if Hilda decides she can live on a retirement income of $40,000 instead of $45,000? After all, she’ll no longer be stuffing her 401(k) or paying Social Security and Medicare taxes (7.65% of each and every paycheck), and her income-tax bill will drop, too. Maybe she’ll also have eliminated her mortgage by then.

Dropping her annual income requirements by $5,000 adds 51 months (4.3 years) to her portfolio’s estimated lifespan. Not huge, but not negligible, either. However, Hilda feels this would be cutting costs a little too close. She wants to look at other possibilities.

Retire Later
What happens when Hilda continues to save just 10% of her income but retires at 68 rather than 65? In that case, she’d have three more years of saving, a higher Social Security benefit (because of her higher lifetime earnings and her beginning benefits later), and she’d need her money to last just 324 months.

In this case, her money would last 255 months, or 21.3 years. By retiring three years later, she’s doubled the longevity of her portfolio. But it still won’t last until age 95. However, if she retires just one more year later — at 69 — the calculator estimates her money will last longer than she will, which is the goal of any retirement plan.

Work in Retirement
Unfortunately, Hilda can’t stand the thought of working full-time for more than another decade. However, she’s open to the idea of working part-time for the first five years of her “retirement.” If she earns $30,000 in each of those five years, her portfolio’s life expectancy improves from 127 months to 237 months, or almost 20 years. That doesn’t get her to age 95, but it’s a significant improvement. In fact, for every year she works part-time in retirement, she adds about two years to the estimated endurance of her portfolio.

Quick note: Because Hilda’s “full retirement age” for Social Security purposes is 66, she shouldn’t begin taking Social Security until then if she’s still working. When you begin benefits before your full retirement age but then earn work-related income, your benefit can be significantly reduced.

Tap Home Equity
There are a few ways to use home equity to boost your retirement. Let’s see how Hilda could add these to her calculations.

First, let’s assume that she no longer needs her family-sized home. She actually has some equity in the home, so she sells it, buys a smaller home, and comes out with an extra $50,000. Realistically, given the state of real estate these days, it would take at least a year to sell her house and actually get that $50,000 into her hands to invest. If it earns 8% a year, it would add 58 months — almost five years — to the longevity of her savings. That’s a decent-sized boost, and that’s not counting the lower cost of heating, cooling, maintaining, and paying property taxes on a smaller home.

The other option is a reverse mortgage, which is when a bank pays you money based on the value of your home, and you don’t pay it back until you move. A reverse mortgage on a home currently worth $300,000 could provide a check of $1,200 every month that the borrower stays in the house, according to www.reversemortgage.org. Our retirement calculator doesn’t have an input field for reverse mortgage, but since it operates essentially like a pension, that’s where we’ll add the $1,200. Input “30″ in the “Years you will receive payments” field, and check the “First payment adjusted for inflation” button (but not the others). Click on the results and — voila! — Hilda’s retirement is fully funded.

While that’s encouraging, we should mention that it assumes Hilda’s mortgage is paid off before she takes out the reverse mortgage. If she moves before she passes away, she’ll have to pay off the loan. Plus, reverse mortgages can be expensive. So our preference is to put off taking out a reverse mortgage for as long as possible, perhaps using it only in the case of an emergency, such as needing in-home long-term care.

Note: There’s a helpful infographic on reverse mortgage myths which can be useful on seeing if this might be the right choice for you.

Change Your Expiration Date
Of course, Hilda’s original retirement plan is perfect as long as she dies within 127 months of retiring. All jokes aside, it’s worth remembering that we’re playing it very safe by assuming she’ll live to 95. According to the Social Security actuarial tables, only 10.3% of 55-year-old women make it to 95. If Hilda’s not in good health or longevity doesn’t run in her family, she might assume she’ll die at age 90. That doesn’t change how long her portfolio will last, but it does change how long she’ll need it to last.

A Mixture of the Factors
We looked at many variables in isolation, but the best solution for Hilda is to tweak several categories to find a combination of changes that she finds palatable. For example, if Hilda downsizes to a smaller home (resulting in a $50,000 investment a year from now), saves $200 more a month, delays retirement to age 66, and works part-time for the first two years of her retirement, her money will last until she’s 95. Considering all her options, Hilda decides these are adjustments she can live with.

The Bottom Line
Retirement calculators are very handy tools, but they’re not crystal balls. The results are based on many variables — such as inflation, investment returns, and Social Security benefits — that we can’t predict and could turn out worse than expected.

How should you handle this uncertainty? Run your numbers once a year, using updated account balances, savings or cd rates, and benefits projections (for example, put in the estimated Social Security benefit found in the statement you receive in the mail three months before your birthday each year).

Also, different calculators provide different results, so don’t rely on just one. For additional opinions, check out:

Despite their shortcomings, retirement calculators do a good job of estimating the value of one decision over another. For Hilda, the variable that had the biggest impact on her plan was retiring a few years later. But it will be different for other people. As one example, boosting a savings rate from 10% to 15% would have a much bigger payoff for younger investors than it did for Hilda, who was already within a decade of her target retirement date.

What will provide the most power to your plan? There’s only one way to find out. Visit a financial calculator and start plugging away.

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